JIT: Support GT_SELECT on xarch (#78879)

JIT: Support GT_SELECT on xarch (#78879)

* JIT: Support GT_SELECT on xarch

Add support for conditional select to the xarch backend. Contained
relops are not supported yet, so for now if-conversion will only produce
these nodes under stress on xarch.

* Fix optimization

* Support decomposition

* Clean up a bit

* Do not break loongarch build

* Stress

* Fix VN for GT_SELECT

Hit this in a test that uses DOTNET_JitOptRepeat=*.

* Revert "Stress"

This reverts commit c0e97510835b909451bf1098650a03bc59afe6b2.

* Run jit-format

* Avoid TP impact in release builds while only enabled under stress

* Add comment

* Sigh

* Port fixes